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Tornado Season Is Getting Longer, and Warmer Weather Is Likely to Blame

Thank global warming for a roughly two week advance in the Great Plains tornado season.

by Michael Byrne
Sep 16 2014, 9:10pm

Image: Marc Manthey/Flickr

Tornadoes are arriving earlier in Tornado Alley. According to a study out this week in the Geophysical Research Letters, storms are hitting the northern and central Great Plains states a full two weeks earlier than they were a half century ago. 

Two weeks might not seem like a huge period of time, but it's worth considering that the whole season historically only runs about eight weeks, from mid-May through mid-July. And while the cause behind the expanded season isn't completely settled, it seems likely that it has to do with warmer average temperatures, e.g. climate change.

Tornado Alley isn't a technical term. It refers generally to the region of the United States where the strongest tornados occur most frequently. Texas is the clear leader, due to its size, but Kansas and Oklahoma boast the highest frequency of strong storms per square-mile. 

Some definitions of the Alley have it extending northward into the Dakotas and across Iowa to the Illinois border. Recent research suggests that the Alley is expanding northward into the plains of Saskatchewan, the result of, yep, warming.

The current study examined National Weather Service data from 1954 to 2009 for northern Texas, Oklahoma, Kansas, and Nebraska. The researchers broke that span into 10 year chunks and analyzed the changes in storm frequency from period to period. For all four states, they found an average seasonal expansion of 1.55 days per 10 year span. Excluding data from Nebraska, this rate of expansion nearly doubled.

"Results are largely unrelated to large-scale climate oscillations, and observed climate trends cannot fully account for observations, which suggest that changes to regional climate dynamics should be further investigated," the paper reports.

Image: NWS

Tornadoes are the products of clashing air temperatures, which is why the season occurs when it does. As Great Plains climates adjust in the springtime, warm, moist air from the south clashes with cold, dry air from the north. The result can be a brutal variety of storm characterized by convection, or the rapid rising of wet, warm air. The result is often intense hail and powerful tornadoes.

A strong jet stream can make the situation even more fertile for tornado production, as this strong blast of high-altitude wind rushes over the top of weaker, warmer surface air. The result is a churning, twisting in the atmosphere, a choice ingredient for the production of strong storms.

So in Tornado Alley we have a big confluence of violent weather patterns, where opposing air masses meet with powerful, cold atmospheric winds rushing down from the Rocky Mountains on the back of the jet stream.

The jet stream is where climate change might come into the equation. Warmer winter and spring seasons—as have been clearly observed over recent years—are likely to push the jet stream to more northern latitudes from where it usually hangs out. This could draw out tornado activity earlier in the year, noted Greg Carbin, a NWS meteorologist not affiliated with the current study, in a statement.

The study outlines another likely climate change connection: El Niño. Particularly in Oklahoma, the warming Pacific waters associated with the El Niño circulation lead to unusual changes to surface air pressures and increased atmospheric turmoil. Basically, El Niño enhances a couple of things that tornadoes love: atmospheric convection (that churning, twisting upward rise) and warm, wet air from the tropical Pacific. In an El Niño year, both intrude deeper into the central US.

While El Niño has been suppressed in recent years, leading to the southwest's devastating droughts, the climate changed future should bring a rough doubling of "extreme" El Niño events, according to a 2013 study in Nature.

"The relationship we do see in Oklahoma is a light but significant connection to El Niño," said Paul Stoy, one of the authors behind the current study and an environmental science professor at the University of Montana. "This makes one suspect that if global climate change is changing these larger circulations, then there is a connection between a global [variability] and tornado activity."